PROJECT ABSTRACT Older adults struggle to maintain newly initiated levels of physical activity (PA) or sedentary behavior (SB) and often regress back to baseline levels over time. This is in part because health behavior theories that inform interventions rarely address how the changing contexts of daily life influence the processes regulating PA and SB, or how those processes differ across the behavior change continuum from adoption to maintenance. These are necessary considerations as PA and SB occur within and across days with optimal levels of these behaviors ideally maintained across the lifespan and are in part driven by temporal and situational cues that rapidly change over time. To date, little research has focused on motivational processes that regulate the dynamic nature of PA and SB adoption and maintenance on micro timescales (i.e., across minutes, hours, or days). Thus, we propose to determine the motivational processes that regulate behavioral adoption versus maintenance over micro timescales, using a dual-process framework combined with Ecological Momentary Assessment (EMA) and sensor-based monitoring of behavior. Older adults identified as PA adopters (n=100) and PA maintainers (n=100) will complete 3 data collection waves over 1 year, with each wave lasting 14 days. EMA questionnaires will be randomly delivered via smartphone 10 times/day on select days in each wave and assess reflective (e.g., evaluating one?s efficacy, exerting self-control) and reactive processes (e.g., contextual cues) within the dual process framework. We will seek to recruit men and women with a racial/ethnic composition that reflects the demographics in our region. Our overarching hypothesis is that reflective processes will be more strongly linked to behavior among adopters, whereas reactive processes will be more strongly linked to behavior among maintainers. Specific Aim 1 will determine the extent to which momentary reflective and reactive processes are differentially associated with subsequent PA and SB among behavioral adopters and maintainers. Specific Aim 2 will determine the extent to which person-level patterns in reflective and reactive processes predict behavioral adoption versus maintenance at each wave and across the entire year. Specific Aim 3 will explore reflective and reactive motivational processes predicting change in adopter/maintainer status from wave to wave. By determining the reflective and reactive motivational processes and person-level patterns in these processes that positively impact PA and SB, we will improve our understanding of the underlying processes that drive health behaviors in real-time. In turn, this will inform future delivery of personalized intervention content under conditions when the content will be most effective to promote sustained behavior change among older adults. Importantly, this R15 will provide training for 2 undergraduate and 2 graduate students; they will be substantively involved in many aspects of the study, such as participant recruitment, screening, and training, monitoring of compliance, equipment initialization and downloading, data cleaning and analysis, and preparing results for publications and presentations.